500 



MEMOIRS OF THE CARNEGIE MUSEUM. 



1. The bcak-sciilpture has a smaller number (ten to twelve) of radial bars, 

 the bars having the same length (5 to 7 mm.) 



2. The outline of the shell is more frequently subellii)tical, often rather regularly 

 so, with both upper and lower margins almost equally curved. In some specimens, 

 indeed, the lower margin is more nearly straight, but this never causes a distinctly 

 humped shape (so often seen in simillimus). No trace of biangulation posteriorly. 



3. Epidermis in young shells bronzy-brown or bronzy-green; in older ones it 

 becoming a deep chestnut-color, inclining partly to blackish. 



In size this species stands between wiilator and vicarius (maximum length 

 73 mm.). The relative dimensions are very much like those of the other three 

 species. The hinge-teeth most resemble those of simillimus, never being stumpy, 

 as in imitator; they are not much cut up; and the posterior pseudocardinal in the 

 left yalve has a strong tendency to disappear, being sometimes entirely missing. 

 The nacre is whitish, but generally lurid (purplish gray) in the cavity. 



Measurements. 



No sexual differences in the shell. 



Anatomtj. — Aside from three very young specimens, which probably are a male 

 and two females, I have six males of good size, and one barren female, a gravid 

 female with eggs, and two females with glochidia (in one of them immature). 



The anatomy is similar to that of D. imitator, simillimus, and vicarius, but 

 the marsupium (PI. XLV, fig. 4b) is quite different, essentially differing from that of 

 the other species. It is located in the middle of the inner gill, leaving between one- 

 fifth and one-fourth of the gill at the anterior end, and about one-third of it at the 

 posterior end non-marsupial. Thus the marsuiiial ]iortion is rather large, and more 

 of it is anterior rather than posterior to the middle. This location comes nearer 

 to what is seen in D. imitator, than in the other two species. The most striking 

 character, however, which we have not before encountered, is that the interlaminar 

 connections of the marsupial part are not interrupted, but form continuous septa, 

 running from near the base of the gill close to the margin. These septa are heavy 

 and stand very closely, forming regular, isolated water-tubes (See section on PI. 

 XLVII, fig. 7). The whole of the marsupium has this structure, and no inter- 



